The contract means CBS Outdoor will continue to sell the advertising on the inside and outside of the bus fleets belonging to the two operators.
According to CBS Outdoor, Transdev London United and London Sovereign represent 10% of the London bus market and carry 195 million passengers every year.
Jason Cotterrell, commercial director at CBS Outdoor, said the renewed partnership will provide brands with exposure in proximity locations and will allow CBS Outdoor to "promote the bus advertising medium with a single voice to compete fiercely for advertising revenue in London".
London United has a longstanding relationship with CBS Outdoor and Richard Casling, deputy managing director and finance director at London United, said the company has "always found the partnership to be operationally successful and financially beneficial".
In January CBS Outdoor hired ex-Toyota marketing director Mike Moran as its new UK managing director. Moran, who started on 19 January, oversees the outdoor media owner's relationships with brand advertisers and stakeholders, reporting to international chief executive Antonio Alonso.
Many of Moran's duties were handled by managing director of sales and marketing Tim Bleakley, before he left CBS Outdoor in October 2009.
Outdoor
CBS Outdoor retains £12m London bus deal
LONDON - Outdoor media owner CBS Outdoor has signed a new £12m five-year contract to sell the advertising on Transdev London United and London Sovereign buses.